This invention relates to a wireless digital transmission system for loudspeakers.
Some wireless loudspeaker systems are known in which an analog audio signal is converted into a frequency modulated signal, this frequency modulated signal being transmitted over the alternating current feeders of a household network. The signal received by the domestic network is then reconverted into an audio signal after extraction of the modulated frequency signal.
Such a teaching is disclosed in particular by U.S. Pat. No. 4,829,570. This patent further envisions the use of a compression device to make it possible to compress analog signals delivered by a compact disc reader whose wide dynamic range requires a very wide passband to make the frequency modulated transmission possible. The wide band and the significant deviations pose numerous problems that are solved in this document by the use of a compression circuit to reduce the total dynamic range of the audio signal. This document makes it possible for us already to become aware of a first difficulty, which is the limitation of stereophonic systems, especially using frequency modulation and operating with analog systems such as variable frequency oscillators.
When it is desired to improve simple stereophonic quality to stereophonic quality of the “digital CD” type, the amount of data to be transmitted is such that the passband very quickly limits the frequency modulation.
Finally, this type of system taught by U.S. Pat. No. 4,829,570 is acceptable for use for private purposes on the domestic network of a personal residence but can be difficult to implement in a building or even less in communities or commercial groupings. In fact, the music broadcast on the feeder network will be picked up at the same instant by all the loudspeakers installed and connected to the network. This poses a problem in the payment of royalties and it is thus desirable to provide a device that makes it possible to avoid general distribution.
Finally, such a device requires, to have the two stereophonic channels, providing a first carrier frequency for the first channel and a second carrier frequency for the second channel. These frequencies will have to be selected according to very precise conditions, which will also limit the passband possibilities.